The mechanism of T cell-B cell cooperation will be studied using soluble factors prepared from activated T cells which enhance in vitro antibody responses. The factor or factors are prepared from cell-free supernatants of mixed cultures of T cells activated to histocompatibility antigens and appropriate target cells. Addition of supernatants to cultures of normal or T cell depleted mouse spleen cells causes nonspecific enhancement of plaque forming cell responses and triggered blastogenesis. The source, properties, and range of effects of the factor will be determined. We will seek evidence on the mechanism of the enhancing effect and hope to determine the relevance of the phenomenon to in vivo immune response induction and expression. In vivo models for the study of the factor will be sought.